On-the-fly beam path error correction for memory link processing

ABSTRACT

Laser beam positioners ( 300, 340 ) employ a steering mirror ( 236, 306 ) that performs small-angle deflection of a laser beam ( 270 ) to compensate for cross-axis ( 110 ) settling errors of a positioner stage ( 302 ). A two-axis mirror is preferred because either axis of the positioner stages may be used for performing work. In one embodiment, the steering mirror is used for error correction only without necessarily requiring coordination with the positioner stage position commands. A fast steering mirror employing a flexure mechanism and piezoelectric actuators to tip and tilt the mirror is preferred in semiconductor link processing (“SLP”) applications. This invention compensates for cross-axis settling time, resulting in increased SLP system throughput and accuracy while simplifying complexity of the positioner stages because the steering mirror corrections relax the positioner stage servo driving requirements.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application No.60/269,646, filed Feb. 16, 2001.

TECHNICAL FIELD

This invention relates to laser processing of circuit links and, inparticular, to a laser system and method employing a laser beam andsubstrate positioning system that incorporates a steering mirror tocompensate for stage positioning errors and enhance link severingthroughput.

BACKGROUND OF THE INVENTION

Yields in integrated circuit (“IC”) device fabrication processes oftenincur defects resulting from alignment variations of subsurface layersor patterns or particulate contaminants. FIGS. 1, 2A, and 2B showrepetitive electronic circuits 10 of an IC device or workpiece 12 thatare typically fabricated in rows or columns to include multipleiterations of redundant circuit elements 14, such as spare rows 16 andcolumns 18 of memory cells 20. With reference to FIGS. 1, 2A, and 2B,circuits 10 are also designed to include particular laser severablecircuit links 22 between electrical contacts 24 that can be removed todisconnect a defective memory cell 20, for example, and substitute areplacement redundant cell 26 in a memory device such as a DRAM, anSRAM, or an embedded memory. Similar techniques are also used to severlinks to program a logic product, gate arrays, or ASICs.

Links 22 are designed with conventional link widths 28 of about 2.5microns, link lengths 30, and element-to-element pitches(center-to-center spacings) 32 of about 8 microns from adjacent circuitstructures or elements 34, such as link structures 36. Although the mostprevalent link materials have been polysilicon and like compositions,memory manufacturers have more recently adopted a variety of moreconductive metallic link materials that may include, but are not limitedto, aluminum, copper, gold nickel, titanium, tungsten, platinum, as wellas other metals, metal alloys such as nickel chromide, metal nitridessuch as titanium or tantalum nitride, metal suicides such as tungstensilicide, or other metal-like materials.

Circuits 10, circuit elements 14, or cells 20 are tested for defects.The links to be severed for correcting the defects are determined fromdevice test data, and the locations of these links are mapped into adatabase or program. Laser pulses have been employed for more than 20years to sever circuit links 22. FIGS. 2A and 2B show a laser spot 38 ofspot size diameter 40 impinging a link structure 36 composed of a link22 positioned above a silicon substrate 42 and between component layersof a passivation layer stack including an overlying passivation layer 44(shown in FIG. 2A but not in FIG. 2B) and an underlying passivationlayer 46 (shown in FIG. 2B but not in FIG. 2A). FIG. 2C is a fragmentarycross-sectional side view of the link structure of FIG. 2B after thelink 22 is removed by the laser pulse.

FIG. 3 is a plan view of a beam positioner travel path 50 performed by atraditional link processing positioning system. Because links 22 aretypically arranged in rows 16 and columns 18 (representative ones shownin dashed lines), the beam position and hence the laser spots 38 arescanned over link positions along an axis in a first travel direction52, moved to a different row 16 or column 18, and then scanned over linkpositions along an axis in a second travel direction 54. Skilled personswill appreciate that scanning may include moving the workpiece 12,moving the laser spot 38, or moving the workpiece 12 and the laser spot38.

Traditional positioning systems are characterized by X-Y translationtables in which the workpiece 12 is secured to an upper stage that movesalong a first axis and is supported by a lower stage that moves along asecond axis that is perpendicular to the first axis. Such systemstypically move the workpiece relative to a fixed beam position or laserspot 38 and are commonly referred to as stacked stage positioningsystems because the lower stage supports the inertial mass of the upperstage which supports workpiece 12. These positioning systems haveexcellent positioning accuracy because interferometers are typicallyused along each axis to determine the absolute position of each stage.This level of accuracy is preferred for link processing because thelaser spot size 40 is typically only a little bigger than link width 28,so even a small discrepancy between the position of laser spot 38 andlink 22 can result in incomplete link severing. In addition, the highdensity of features on semiconductor wafers results in small positioningerrors potentially causing laser damage to nearby structures. Stackedstage positioning systems are, however, relatively slow because thestarting, stopping, and change of direction of the inertial mass of thestages increase the time required for the laser tool to process all thedesignated links 22 on workpiece 12.

In split-axis positioning systems, the upper stage is not supported by,and moves independently from, the lower stage and the workpiece iscarried on a first axis or stage while the tool, such as a fixedreflecting mirror and focusing lens, is carried on the second axis orstage. Split-axis positioning systems are becoming advantageous as theoverall size and weight of workpieces 12 increase, utilizing longer andhence more massive stages.

More recently, planar positioning systems have been employed in whichthe workpiece is carried on a single stage that is movable by two ormore actuators while the tool remains in a substantially fixed position.These systems translate the workpiece in two dimensions by coordinatingthe efforts of the actuators. Some planar positioning systems may alsobe capable of rotating the workpiece.

Semiconductor Link processing (“SLP”) systems built by ElectroScientific Industries, Inc. (“ESI”) of Portland, Oreg. employ on-the-fly(“OTF”) link processing to achieve both accuracy and high throughput.During OTF processing, the laser beam is pulsed as a linear stage beampositioner passes designated links 12 under the beam position. The stagetypically moves along a single axis at a time and does not stop at eachlink position. The on-axis position of beam spot 38 in the directiontravel 52 does not have to be accurately controlled; rather, itsposition is accurately sensed to trigger laser spot 38 to hit link 22accurately.

In contrast and with reference again to FIG. 3, the position of beamspot 38 along cross-axes 56 or 58 is controlled within specifiedaccuracy as the beam positioner passes over each link 22. Due to theinertial mass of the stage or stages, a set-up move to start an OTF runproduces ringing in the cross-axis position, and the first link 22 in anOTF run cannot be processed until the cross-axis position has settledproperly. The settling delay or setting distance 60 reduces processingthroughput. Without a settling delay (or, equivalently, a buffer zone ofsettling distance 60) inserted before the first laser pulse, severallinks 22 would be processed with serious cross-axis errors.

Although OTF speed has been improved by accelerating over gaps in thelink runs, one limiting factor on the effectiveness of this “gapprofiling” is still the requirement for the cross axis to settle withinits specified accuracy. At the same time, feature sizes, such as linklength 30 and link pitch 32, are continuing to decrease, causing theneed for dimensional precision to increase. Efforts to further increasethe performance of the stage or stages substantially increase the costsof the positioning system.

The traditional way to provide for two-axis deflection of a laser beamemploys a high-speed short-movement positioner (“fast positioner”) 62,such as a pair of galvanometer driven mirrors 64 and 66 shown in FIG. 4.FIG. 4 is a simplified depiction of a galvanometer-driven X-axis mirror64 and a galvanometer-driven Y-axis mirror 66 positioned along anoptical path 70 between a fixed mirror 72 and focusing optics 78. Eachgalvanometer-driven mirror deflects the laser beam along a single axis.U.S. Pat. No. 4,532,402 of Overbeck discloses a stacked stage beampositioning system that employs such a fast positioner, and U.S. Pat.Nos. 5,751,585 and 5,847,960 of Cutler et al. disclose split-axis beampositioning systems in which the upper stage(s) carry at least one fastpositioner. Systems employing such fast positioners are used for nonlinkblowing processes, such as via drilling, because they cannot currentlydeliver the beam as accurately as “fixed” laser head positioners.

The split-axis nature of such positioners may introduce rotational Abbeerrors, and the galvanometers may introduce additional positioningerrors. In addition, because there must be separation between the twogalvanometer-controlled mirrors, the mirrors cannot both be located nearthe entrance pupil to the focusing optics. This separation results in anoffset of the beam that can degrade the quality of the focused spot.Moreover, two-mirror configurations constrain the entrance pupil to bedisplaced farther from the focusing optics, resulting in an increasedcomplexity and limited numerical aperture of the focusing optics,therefore limiting the smallest achievable spot size. Even assuming suchpositioners could be used for link-severing, the above-described spotquality degradation would cause poor quality link-severing or incompletelink-severing and result in low open resistance across severed links 22.

What is still needed, therefore, is a system and method for achievinghigher link-processing throughput while maintaining focused spotquality.

SUMMARY OF THE INVENTION

An object of the invention is, therefore, to provide a system and/ormethod for achieving higher link-processing throughput while maintainingfocused spot quality.

Another object of the invention is to employ a two-axis steering mirrorto correct for linear stage settling errors.

Yet another object of the invention is to provide a positioner systememploying coordinated motion for semiconductor link processingapplications.

This invention preferably employs a two-axis steering mirror, pivotallymounted at the entrance pupil of the focusing lens, to performsmall-angle motions that deflect the laser beam enough to compensate forcross-axis settling errors on the order of tens of microns. Although thesettling errors occur in both axes, one embodiment of this invention isconcerned primarily with correcting settling errors in a cross-axisdirection to the OTF direction of linear stage travel. A two-axissteering mirror is employed for these corrections because either axis ofthe linear stage may be used as the OTF axis. The beam steering mirroris preferably used for error correction only and does not requirecoordination with or modification of the linear stage position commands,although such coordination is possible.

At least three technologies can be used to tilt a mirror in two axesabout a single pivot point. These technologies include fast steeringmirrors (“FSMs”) that employ a flexure mechanism and voice coilactuators to tilt the mirror, piezoelectric actuators that rely upondeformation of piezoelectric materials to tilt a mirror, and deformablemirrors that employ piezoelectric or electrostrictive actuators todeform the surface of the mirror. Piezoelectric actuators are preferred.

Advantages of the invention include the elimination of cross-axissettling time, resulting in increased throughput particularly for SLPsystems. The invention also facilitates improved manufacturability ofthe main positioning stage(s) due to relaxed servo performancerequirements because the steering mirror can correct for linear stageerrors.

Additional objects and advantages of this invention will be apparentfrom the following detailed description of preferred embodiments thereofwhich proceed with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a portion of a DRAM showing theredundant layout of and programmable links in a spare row of genericcircuit cells.

FIG. 2A is a fragmentary cross-sectional side view of a conventional,large semiconductor link structure receiving a laser pulse characterizedby prior art pulse parameters.

FIG. 2B is a fragmentary top view of the link structure and the laserpulse of FIG. 2A, together with an adjacent circuit structure.

FIG. 2C is a fragmentary cross-sectional side view of the link structureof FIG. 2B after the link is removed by the prior art laser pulse.

FIG. 3 is a plan view of a prior art beam travel path.

FIG. 4 is a simplified side view of a prior art fast positioneremploying a pair of galvanometer-driven mirrors that deflect the laserbeam along different respective single axes.

FIG. 5 schematically illustrates a side sectional view of a preferredtwo-axis mirror employed in the practice of the invention.

FIG. 6 schematically illustrates a partial front view of a preferredtwo-axis mirror employed in the practice of the invention.

FIG. 7 illustrates the effect of the steering mirror during the OTF run.

FIG. 8 illustrates an exemplary multi-row, cross-axis dithering(“MRCAD”) work path.

FIG. 9 is a side sectional view of a representative two-axis steeringmirror.

FIG. 10 is a simplified plan view of a representative two-axis steeringmirror.

FIG. 11 is a simplified schematic block diagram of an exemplarypositioner control system for coordinating the stage positioning and thesteering mirror for error correction.

FIG. 12 is a simplified schematic block diagram of an exemplarypositioner control system for coordinating the stage positioning and thesteering mirror for beam-to-work scans and error correction.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of a representative beam positioning system is describedin detail in U.S. Pat. No. 4,532,402 of Overbeck, which is assigned tothe assignee of this application. A preferred X-Y stage is a “Dynamix”model available from Newport Corporation of Irvine, Calif.

The beam positioning system preferably employs a laser controller thatcontrols a stacked, split-axis, or planar positioner system andcoordinates with reflectors to target and focus laser system output to adesired laser link 22 on IC device or workpiece 12. The beam positioningsystem permits quick movement between links 22 on the same or differentworkpieces 12 to effect unique link-severing operations based onprovided test or design data. The beam positioning system mayalternatively or additionally employ the improvements, beam positioners,or coordinated motion schemes described in U.S. Pat. Nos. 5,751,585,5,798,927, and 5,847,960 of Cutler et al., which are assigned to theassignee of this application. Other fixed head or linear motor drivenconventional positioning systems could also be employed, as well as thesystems employed in the 9000, 9800, and 1225 model series manufacturedby ESI of Portland, Oreg., the assignee of this application.

With reference to FIGS. 5 and 6 and with respect to this invention, thefinal turn mirror of a fixed head system or alternatively fastpositioner 66 (FIG. 4) is preferably replaced by a single high-speed,high-accuracy two-axis steering mirror system 100 that includes a mirror102 capable of actuation with at least two degrees of freedom. Mirror102 has a centrally positioned pivot point 104 that preferably coincideswith an entrance pupil 106 of a focusing lens 108. Two-axis steeringmirror system 100 is preferably used for error correction, although itmay be employed for beam steering because either axis of the linearstage maybe used as the OTF axis.

Because the beam is focused to a very fine spot size for SLPapplications, the mechanism directing mirror system 100 preferablypivots the mirror 102 along at least two axes about pivot point 104,which is located at or near the entrance pupil of focusing optics orlens 108. Small angle perturbations in the position of mirror 102deflect the beam enough to correct for linear stage settling errors atthe work surface, and because mirror 102 is located at or near theentrance pupil of focusing lens 108, the beam is shifted withoutdistorting the focused spot, allowing delivery of a small, high qualityspot.

In one embodiment, settling errors in a cross-axis direction 110 arecorrected by mirror 102, while motion in an on-axis direction 112 is notcorrected. This single axis correction allows the linear stageinterferometer feedback to be the sole source of laser pulse triggering.However, with proper coordination, on-axis direction 112 steering mirror102 motion is possible, although it complicates the design andintroduces additional error sources that can degrade on-axis direction112 accuracy if such errors are not addressed.

Motion in each axis of mirror 102 exhibits scale factor and offseterrors, noise, and cross-axis coupling. These error source can bewell-controlled and calibrated out in the system, with noise andtemperature stability effects controlled through conventional designtechniques.

Calibration of mirror system 100 through beam-to-work (“BTW”) alignmentscan correct for any non-linearity and alignment errors in steeringmirror 102. Traditionally, the term beam-to-work is used as nomenclaturefor the process of scanning the linear stage back and forth, whiledirecting the laser beam spot at low power at an alignment target on thewafer or workpiece 12 (FIG. 1). Optical measurements of the reflectionoff the target are used to precisely determine target and hence waferlocation. By scanning several targets with BTW scans, the offset androtation of the wafer relative to the beam spot can be ascertained. Itis also possible to map out other effects such as axis orthogonality andpositional distortions.

After mirror system 100 is added to the laser system, traditional BTWtype scans can be used to map out any inaccuracies/nonlinearities insteering mirror 102 response. This is accomplished by doing a BTW scanwith mirror 102 in the nominal zero offset (in either axis) position.Then mirror 102 is tilted, and another BTW scan is performed todetermine how much lateral offset of the laser beam spot is imparted bythe tilt. By measuring the offset caused by numerous mirror tilts in theU and V axes, mirror system 100 can be fully characterized.

Once the response of mirror system 100 is determined to sufficientlyfine precision, then instead of moving the linear stage back and forth,it is possible to use mirror system 100 for subsequent BTW typealignment scans.

FIG. 7 illustrates the corrective effect of two-axis steering mirrorsystem 100 during an OTF run. A linear stage ringing is represented by aringing curve 120. Mirror 102 deflects the laser beam in cross-axisdirection 110 as represented by a correction curve 122 that is theinverse of ringing curve 120. The resulting beam position is the sum ofthe linear stage motion and the deflected beam position and isrepresented by a resulting beam path curve 124, which is free ofcross-axis error.

FIG. 8 illustrates using steering mirror system 100 for MRCAD processingduring boustrophedon or raster scanning in the context of link severingto further improve the speed at which links are blown. In a preferredmode of operation, MRCAD scanning is done in cross-axis direction 110while moving along a row 130 of links 132. MRCAD scanning employssteering mirror 102 (FIGS. 5 and 6) to direct the laser beam along apathway 134 at links 132 and adjacent links 136 in adjacent rows 138without needing to move the slower linear motion stage in cross-axisdirection 110. This is possible because not all the links in each rowneed to be blown. Link processing becomes far more efficient with MRCADbecause the linear or stages do not have to be scanned or slewed downeach row, so the total number of link row scans can be substantiallyreduced. As integration increases and link sizes, spot sizes, and pitchdistance decrease, MRCAD scanning will become an even more valuabletechnique.

In another mode, supplemental on-axis dithering (“SOAD”) uses mirror 102to deflect the beam in on-axis direction 112 (FIGS. 5-7). In thisoperational mode, the beam can be quickly directed ahead in on-axisdirection 112, severing links while the linear motion stage catches up.The SOAD scan ahead or scan behind the stage feature allows thepositioning system to reduce stage velocity changes or to sever severallinks during a single slowed movement segment.

At least three technologies can be employed to tilt mirror 102 in twoaxes about pivot point 104. These technologies include FSMs that employa flexure mechanism and voice coil actuators, piezoelectric actuatorsthat rely upon deformation of piezoelectric materials, and piezoelectricor electrostrictive actuators to deform the surface of a mirror.Suitable voice coil actuated FSMs are available from Ball AerospaceCorporation of Broomfield, Colo. and Newport Corporation of Irvine,Calif. However, the preferred actuator is a model S-330 Ultra-Fast PiezoTip/Tilt Platform manufactured by Physik Instrumente (“PI”) GmbH & Co.of Karlsruhe, Germany.

Traditional galvanometers are not typically used for this applicationbecause they each tilt a mirror about only one axis and ordinarily haveinsufficient positioning accuracy. Moreover, a pair of physicallyseparated galvanometer mirrors is required for two axes of actuation.This separation is incompatible with the desire that actuation occurabout one pivot point located near the entrance pupil of focusing lens108 (FIGS. 5 and 6) to maintain a high quality laser spot at the surfaceof workpiece 12. Nevertheless, it is possible to employ galvanometerdeflected mirrors in this invention, particularly if employed insingle-axis and small deflection applications to maintain accuracy andwell focused laser spots.

By way of example only, FIGS. 9 and 10 show an FSM two-axis mirrorsystem 200 in which four electrical to mechanical vibration generatorsor transducers are supported by a transducer support platform 220 in aquadrature relationship, whereby transducers 222, 224, 226, and 228 arepositioned at 0, 90, 180, and 270 degrees with respect to a central axis230; therefore, adjacent ones of the transducers are set at right angleswith respect to each other. A movable mirror support member 232 has acentral portion or hub 234 bearing a mirror or reflective surface 236centered with respect to axis 230. Mirror 236 has a diameter of about 30mm or less to reduce its weight and facilitate high frequency responsefor desired beam correction. Mirror 236 is coated with conventionallaser optical coatings to account for laser wavelength or designparameters.

Four lightweight rigid struts or elongated members 242, 244, 246, and248 extend radially from hub 234 of mirror support member 232, and haverespective peripheral terminal portions 252, 254, 256, and 258 affixedto respective transducers 222, 224, 226, and 228, which are electricallymovable voice coils. For a further description of a suitableconventional voice coil/loudspeaker arrangement, see Van Nostrand'sScientific Encyclopedia, Sixth Edition, page 1786. The use of suchconventional loudspeaker coils for the transducers to perform mechanicalactuation, decreases the manufacturing cost of the apparatus. Thefloating mirror support 232 can beneficially be made of a lightweightmaterial, such as metal (e.g. aluminum or beryllium) or plastic,enabling rapid response to the electrical input signals to the voicecoils to be described.

A tip control generator 260 is connected to transducers 224 and 228 tocause them to move in a complementary “push pull” relationship with eachother. Similarly, a tilt control generator 262 is connected totransducers 222 and 226 to cause these coils to also move in acomplementary push pull relationship with each other. A laser beam 270is reflected off reflective surface 236 and a reflected beam 272 ispositioned by the generators controlling the cross axis, which isperpendicular to OTF direction of travel, to compensate for cross axiserrors. The pairs of signals produced by each generator assume apush-pull relationship, so that when transducer 222 is pulling upperterminal portion 252 of support member 232 to the right in FIG. 10,lower transducer 226 is pushing terminal portion 256 to the left, totilt reflective surface 236, thereby deflecting reflected beam 272. Theactuation can be alternated at the beginning of an OTF run, for example,to move reflective surface 236 at a proper frequency and dampedamplitude to compensate for linear stage ringing in cross-axis direction110, thereby eliminating the negative effects of linear stage settlingtime and producing a relatively straight beam path. Thus, links thatwould otherwise be in the conventional buffer zone can be processedaccurately.

Mirror systems suitable for use with this invention can be implementedwith a large enough field to do MRCAD scans by providing beam deflectionin a range of about 50 to 100 microns; however, such mirror systems canalso be implemented for cross-axis correction only by providing beamdeflection in a range of about 10 to 50 microns or as little as about 10to 20 microns. The mirror is preferably positioned within about plus orminus 1 mm of the entrance pupil of the focusing lens. These ranges areexemplary only and can be modified to suit the system design andparticular link processing applications.

The preferred model S-330 Tip/Tilt Platform manufactured by PI usespiezoelectric actuators for high speed, two-dimensional mirror tilting.Strain gage sensors accurately determine mirror position and providefeedback signals to the control electronics and drive circuitry. A morecomplete description of the model S-330 Tip/Tilt Platform is availableat the PI web site, www.physikinstrumente.com.

The main advantages of the PI Piezo Tip/Tilt Platform are that thedevice is commercially available and has a very compact size thatreadily mounts in an ESI model 9820 positioning system.

Disadvantages of the PI Piezo Tip/Tilt Platform are that it hasinsufficient beam deflection range for use in beam-to-work scanningapplications even though its range is sufficient for error correctionapplications; and nonlinear motion, thermal drive, hysteresis, andhigh-voltage actuation are all inherent problems with piezoelectricactuation that have to be accounted for.

Of course, other vendors or other types of mirror or actuator designsare suitable for use with this invention.

In addition to all the other above-described advantages, this inventionpermits a relaxation on the requirements for the linear motors (jerktime, settling time) using the secondary system to correct for errors.This substantially reduces the cost of the linear motors and alsoreduces the dependency of the system throughput on the accelerationlimit of the linear stage or stages.

FIG. 11 shows an embodiment of a positioner control system 300 of thisinvention for coordinating the positioning of X- and Y-axis motionstages 302 and 304, and also the positioning of a two-axis steeringmirror 306 for positioning error correction. Of course, motion stages302 and 304 may be combined into a single planar motion stage havingpositioning control in the X- and Y-axis directions. In a standardoperational mode, two-axis steering mirror 306 is used to correctpositioning errors caused by X- and Y-axis motion stages 302 and 304.

A position command generator 308 generates X- and Y-axis positioncommand signals for delivery through summing junctions 310 and 312 to X-and Y-axis motion controllers 314 and 316 to respective X- and Y-axismotion stages 302 and 304. The actual positions of X- and Y-axis motionstages 302 and 304 are sensed by respective X- and Y-axis positionsensors 318 and 320 and signals representing the actual positions areconveyed to adders or summing junctions 310 and 312 to generate X- andY-axis position error signals. X- and Y-axis motion controllers 314 and316 receive the error signals and act to minimize any errors between thecommanded and actual positions. For high-accuracy applications, X- andY-axis position sensors 318 and 320 are preferably interferometers.

Residual error signals, such as those generated by ringing, are conveyedthrough enabling gates 322 and 324 to a coordinate transformationgenerator 326, which may be optional depending on whether motion stages302 and 304 share a common coordinate system with two-axis steeringmirror 306. In either event, the residual error signals are passedthrough adders or summing junctions 328 and 330 to U- and V-axissteering mirror controllers 332 and 334, which act to tip and/or tiltsteering mirror 306 by controlled amounts to deflect, for example, laserbeam 270 (FIG. 9) to correct for positioning errors of X- and Y-axismotion stages 302 and 304. The actual tip and/or tilt positions oftwo-axis steering mirror 306 are sensed by respective tip and tiltsensors 336 and 338 and signals representing the actual tip and tiltpositions are conveyed to adders or summing junctions 328 and 330 togenerate tip and tilt position error signals. U- and V-axis steeringmirror controllers 332 and 334 receive the error signals and act tocorrect any errors between the commanded and actual positions. Forhigh-accuracy applications, two-axis steering mirror 306 is preferably apiezoelectric tilt/tip platform, and position sensors 318 and 320 arepreferably strain gages. Suitable alternative sensors may includeoptical, capacitive, and inductive sensing techniques. In thisembodiment, skilled workers will understand that U- and V-axis steeringmirror controllers 332 and 334 should be adapted to provide zero to 100volt drive signals to the piezoelectric actuators deflecting two-axissteering mirror 306.

Enabling gates 322 and 324 implement a provision in which positioncommand generator 308 can selectively disable position error correctionfor either the X or the Y axis, thereby enabling error correction forthe cross-axis while leaving the on-axis unaffected, or vice versa.

FIG. 12 shows an embodiment of a positioner control system 340 forcoordinating the positioning of X- and Y-axis motions stages 302 and 304and, in this embodiment, FSM 236 (FIGS. 9 and 10) for MRCAD scans andpositioning error correction. In an extended operational mode, thesteering mirror is used for error correction and MRCAD scanning. In thismode of operation, a position command generator 342 generates X- andY-axis positioning commands for X- and Y-axis motion stages 302 and 304and also U- and V-axis tip and tilt commands for deflecting FSM 236.Summing junctions 328 and 330 generate the positioning command for FSM236 as the sum of the error signals from X- and Y-axis motion stages 302and 304 and, in this embodiment, also the U- and V-axis tip and tiltcommands.

The error signals are generated in the same manner as in the standarderror correction mode. The additional U- and V-axis tip and tiltcommands are produced by position command generator 342 to accomplishthe desired beam-to-work scanning. Because beam-to-work and MRCADapplications typically require wider ranges of mirror deflection, thisembodiment of the invention preferably employs voice coil actuated FSMtwo-axis mirror system 200.

In typical operation, the position commands for MRCAD scanning are usedto produce cross-axis motion of the laser beam without commandingcross-axis motion of the motion stages. However, other applications areenvisioned that would benefit from on-axis supplemental dithering toboustrophedon scanning.

The control schemes depicted in these figures are intended to illustratethe basic implementation and operation of this invention. More advancedcontrol schemes, such as those employing feedforward commands to themotion stages and steering mirror, will be obvious to those skilled inthe art.

Skilled workers will appreciate that the two-axis steering mirrorsystems of this invention can be adapted for use in etched-circuit boardvia drilling, micro-machining, and laser trimming applications as wellas for link severing.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiment of thisinvention without departing from the underlying principles thereof. Thescope of this invention should, therefore, be determined only by thefollowing claims.

We claim:
 1. An apparatus for directing a laser beam toward a target location on a workpiece in response to target location coordinate position command information, the workpiece having a workpiece surface, comprising: a positioner positioning the workpiece and the laser beam relative to one another in response to the coordinate position command information; first and second position sensors coupled to the positioner for producing first and second position signals indicative of an actual coordinate position of the positioner; processing circuitry implemented to perform comparisons of the coordinate position command information and the first and second position signals and to provide from the comparisons one or more error signals indicative of a difference between the coordinate position command information and the actual coordinate position, the difference including a transient signal component representing laser beam position errors at the workpiece surface; a steering mirror controller system producing a position correction signal in response to each error signal provided; a two-axis steering mirror including a pivot point and positioned to receive the laser beam at or near the pivot point, the two-axis steering mirror, in response to the position correction signal, imparting to the laser beam angular motions that deflect the laser beam in a manner sufficient to compensate for the laser beam position errors; and a focusing lens having an entrance pupil and positioned to receive the deflected laser beam and focus it on the target location of the workpiece, the entrance pupil being set at or near the pivot point to provide a substantially distortion-free deflected laser beam.
 2. The apparatus of claim 1 in which the position correction signal includes first and second position correction signal portions, the steering mirror controller system includes first and second steering mirror controllers, and the one or more error signals include first and second error signals that produce first and second position correction signals to which the first and second steering mirror controllers respond, and the first and second steering mirror controllers producing the respective first and second position correction signal portions to which the two-axis steering mirror responds to deflect the laser beam.
 3. The apparatus of claim 1 in which the coordinate position command information includes information for positioning the positioner to respective X-axis and Y-axis orthogonal coordinate locations.
 4. The apparatus of claim 2 in which the first and second error signals conform to a first coordinate system and the motion of the two-axis steering mirror is characterized with reference to a second coordinate system, and in which the apparatus further includes a coordinate transform generator for converting at least one of the first and second error signals to the second coordinate system.
 5. The apparatus of claim 1 in which the steering mirror controller system includes first and second steering mirror controllers, and in which the target location coordinate position command information includes mirror positioning information, the first and second steering mirror controllers positioning the two-axis steering mirror in response to the mirror positioning information and the position correction signal.
 6. The apparatus of claim 1 in which the two-axis steering mirror is positioned by at least one piezo electric actuator.
 7. The apparatus of claim 1 in which the two-axis steering mirror is positioned by at least one voice coil actuator.
 8. The apparatus of claim 1 in which the position correction signal includes a series of position correction signal components, and in which the positioner scans the workpiece and the laser beam relative to one another in a second axis direction in response to a series of coordinate position command information while the two-axis steering mirror is responsive to the series of position correction signal components to receive the laser beam and deflect it toward a set of the target locations on the workpiece.
 9. The apparatus of claim 1 in which the workpiece includes an integrated memory circuit and in which the target location includes a severable link for removing a defective memory cell.
 10. The apparatus of claim 1 in which the workpiece includes an electronic circuit element that is trimmed to a predetermined performance characteristic by the laser beam.
 11. The apparatus of claim 1 in which the positioner includes stages that are arranged in a stacked configuration.
 12. The apparatus of claim 1 in which the positioner includes stages that are arranged in a split-axis configuration.
 13. The apparatus of claim 1 in which the positioner includes a planar positioning stage.
 14. A method for directing a laser beam toward a target location on a workpiece in response to target location coordinate position command information, the workpiece having a workpiece surface, comprising: positioning the workpiece and the laser beam relative to one another in response to the coordinate position command information; sensing an actual coordinate position of the workpiece relative to the coordinate position command information; producing one or more error signals indicative of a difference between the coordinate position command information and the actual coordinate position, the difference including a transient signal component representing laser beam position errors at the workpiece surface; producing a position correction signal in response to each error signal produced; positioning a two-axis steering mirror including a pivot point for receiving the laser beam at or near the pivot point, the two-axis steering mirror, in response to the position correction signal, imparting to the laser beam angular motions that deflect the laser beam in a manner sufficient to compensate for the laser beam position errors; and providing a focusing lens having an entrance pupil and positioned to receive the deflected laser beam and focus it on the target location on the workpiece, the entrance pupil being set at or near the pivot point to provide a substantially distortion-free deflected laser beam at the workpiece surface.
 15. The method of claim 14 in which the one or more error signals include first and second error signals and the position correction signal includes first and second position correction signal portions, the first and second position correction signal portions produced in response to the respective first and second error signals to position the two-axis steering mirror.
 16. The method of claim 14 which the coordinate position command information includes X-axis and Y-axis orthogonal coordinate locations.
 17. The method of claim 14 in which the one or more error signals conform to a first coordinate system and the motion of the two-axis steering mirror is characterized with reference to a second coordinate system, and in which the method further includes transforming at least one of the error signals into the second coordinate system.
 18. The method of claim 14 in which the target location coordinate position command information includes mirror positioning information, and the method further includes positioning the two-axis steering mirror in response to the mirror positioning information and the position correction signal.
 19. The method of claim 14 in which the position correction signal includes a series of position correction signal components, and further including; scanning the workpiece and the laser beam relative to one another in a second axis direction in response to a series of coordinate position command information; and moving the two-axis steering mirror in response to the series of position correction signal components. 